Hypocrates specifically recommended doctors to boil and filter water before giving to patients to drink. Ancient Egytpians, Indians and Romans understood that

Hypocrates specifically recommended doctors to boil and filter water before giving to patients to drink. Ancient Egytpians, Indians and Romans understood that boiling makes water safer to drink as well (Galen even suggested using boiling water to sterilize surgical instruments). IMHO it makes perfect sense in the context of the miasma theory (if people thought that "bad smells" were the main vector spreading diseases and illnesses and boiling water removes bad smells, improves taste etc. then it also must have made it safer to drink).